1. Field of the Invention
This invention relates to liquid-crystal mixtures having certain resistance values, and to a method of adjusting the specific resistance of liquid-crystal mixtures to prespecified values, typically in the range from 10.sup.9 .OMEGA..multidot.cm to a few 10.sup.12 .OMEGA..multidot.cm. This also applies in particular to liquid-crystal mixtures which comprise or even consist of terminally fluorinated compounds or compounds carrying terminally fluorinated substituents, and in general to high-resistance liquid-crystal mixtures of low polarity.
In particular, the resistance is adjusted using acidic compounds, particularly preferably phenols. The present invention also relates to novel substituted phenols. The present invention furthermore relates to liquid-crystal displays containing liquid-crystal mixtures according to the invention.
2. Description of the Prior Art
Liquid-crystal mixtures having high specific resistance values cause problems in some types of liquid-crystal display.
A major problem of these high-resistance liquid-crystal mixtures is the occurrence of electrostatic charges. This sometimes happens simply when protective films, for example of polarizers, or compensation films are peeled off during production of the displays. However, electrostatic charging can also occur during operation of such displays, for example due to contact and/or friction with plastic parts or cloths. In automobile radio displays, for example, electrostatic charges of this type can even occur due to finger contact with the display. In the simplest case, this electrostatic charge can result in undesired switching on of a switched-off display or part of the display.
Frequently, however, irreversible changes to the display are observed. These are caused, for example, by a change to the alignment layers by the electrostatic charges. This phenomenon is observed in particular in TN and STN displays. In displays of this type, a high specific resistance of the liquid-crystal mixture frequently also results in information which has been displayed for an extended period remaining even after the display has been switched off. Owing to the high specific resistance of the liquid-crystal mixtures, accumulated charge carriers can only be dispersed with difficulty, causing the occurrence of so-called "afterimages" or "ghost images", often also referred to as the "sticking effect" or "image sticking effect". In displays with active matrix addressing (for example TN-AMDs or IPS-AMDs), the active matrix's non-linear switches (for example TFTs), in particular, can be damaged or even destroyed. "Image sticking effects" can also occur in AMDs.
A number of proposals for solving this problem have already been made in the prior art. Besides equipment measures, intended to prevent the occurrence of electrostatic charging in, for example, the production of liquid-crystal displays, a number of suggestions have been described for optimizing the liquid-crystal mixtures.
Most of these suggestions propose the use of various dopants in order to achieve the desired resistance values. However, the solubility of the dopants in the liquid-crystal mixtures is usually problematic here. Furthermore, undesired effects, such as, for example, the lowering of the clearing point and changes in the other physical properties, frequently occur. The reproducibility of the setting of the desired resistance is frequently also not good or the range of achievable resistance values is relatively narrow.
A further essential property of the compounds employed to adjust the conductivity or specific resistance of liquid-crystal mixtures is their vapor pressure. This must not be too high, as must that of the other constituents of the liquid-crystal mixtures, since otherwise a change in the composition can occur, resulting here precisely in an undesired change in the resistance. This is of particular importance in the extremely widespread use of vacuum filling units in display manufacture. The changes which occur seem to be dependent on the duration and magnitude of the pressures which occur.
A typical example of compounds recently used to set certain specific resistance values in liquid-crystal mixtures are crown ethers, used as described in WO 97-03 164. However, even when used in small amounts, these result in very considerable reductions in the resistance.
In addition, owing to the interaction of the crown ethers with various impurities, both in the liquid-crystal mixtures and on the internal display surfaces, i.e. essentially the alignment layers, the results achieved are very highly dependent on the material used, and the reproducibility is frequently inadequate. There was thus a demand for substances for the reproducible adjustment of the specific resistance which are readily soluble in liquid-crystal mixtures and are compatible with many alignment layers.
Compounds of the formulae ##STR2## are known, see, for example, Chemical Abstracts CAS-2894-87-3 and CAS-2200-70-6.
JP-A 08-067 577 proposes tri(polyoxyalkylene)amine for reducing the specific resistance of liquid-crystal mixtures.
JP-A 08-337 778 describes liquid-crystal mixtures containing peroxide-destroying compounds for use in STN displays.